The message ventilate is showing

As pointed out by bansheracer, "Ventilate" means you need to open the valve on the right to let out some steam from the milk steamer wand. This can also happen if you accidentally push the 'steam' button but don't actually let out any steam.
If it is stuck or needs cleaning, unscrew the metal piece and dip the plastic bit in a jug of hot water - it should resolve most of the dried milk in about 5-10 minutes.

Hi
If you can't get water out of the wand, the ventilate won't go away. If you can't get water from the wand you have a blockage in the steam tip and it just needs to cleared and you will good to go. If you can get water out of the wand and you get a forceful stream, your flow meter is bad. If you can get water from the wand but the flow is slow, you have a restriction in the flow meter. It needs to be removed and cleared to work again.